Optically switched sensor array

ABSTRACT

A fiber optic array having a plurality of subarrays of optical sensors, the subarrays being spaced apart along and optically connected to input and output buses through associated input and output optically actuated optical switches in series in the input and output buses. First and second light signal pulse trains are applied to one and other ends of the input bus and a third light signal pulse train is applied to one end of the output bus. The input switches are sequentially responsive to each first light signal pulse on the input bus for passing to an associated subarray a pulse of second light signal that is simultaneously present at that input switch such that second light signal pulses of the same magnitude are applied to each subarray. Associated output switches are sequentially responsive to third light signal pulses for passing trains of second light signal pulses from the arrays onto the output bus and to processing equipment.

BACKGROUND

This invention relates to distributed fiber optic sensor arrays such asare employed in shipboard hydrophone systems for sensing changes inphysical phenomena and more particularly to the provision of a systememploying distributed arrays of optically switched fiber optic sensorarrays.

A conventional fiber optic hydrophone array that is immersed in waterand pulled behind a ship for sensing acoustic vibrations in the watergenerally comprises a plurality of spaced apart (i.e., distributed)fiber optic sensors connected in series or in a ladder configuration orin a matrix configuration. In practice, several hydrophone arrays areconnected in series behind the vessel with a number of input and outputfiber optic buses being required and extending back to processingequipment on the towing vessel for each array or subarray. Such fiberoptic sensor arrays are well known in the art, being described inpublications and patents such as: U.S. Pat. No. 4,632,551, issued Dec.30, 1986 for Passive Sampling Interferometric Sensor Arrays by G. A.Pavlath; U.S. Pat. No. 4,697,926, issued Oct. 6, 1987 for CoherentDistributed Sensor and Method Using Short Coherence Length Sources by R.C. Youngquist, etal; U.S. Pat. No. 4,699,513, issued Oct. 13, 1987 forDistributed Sensor and Method Using Coherence Multiplexing ofFiber-Optic Interferometeric Sensors by Janet L. Brooks, etal; U.S. Pat.No. 4,770,535, issued Sep. 13, 1988 for Distributed Sensor Array andMethod Using a Pulse Signal Source by B. Y. Kim, etal; U.S. Pat. No.4,789,240, issued Dec. 6, 1988 for Wavelength Switched PassiveInterferometric Sensor System by I. J. Bush; U.S. Pat. No. 4,818,064,issued Apr. 4, 1989 for Sensor Array and Method of SelectiveInterferometric Sensing by Use of Coherent Synthesis by R. C.Youngquist, etal; U.S. Pat. No. 4,889,986, issued Dec. 26, 1989 forSerial Interferometric Fiber-Optic Sensor Array by A. D. Kersey, etal;U.S. Pat. No. 5,011,262, issued Apr. 30, 1991 for Fiber Optic SensorArray by M. R. Layton; and U.S. Pat. No. 5,039,221, issued Aug. 13, 1991for Interferometer Calibration for Fiber Optic Sensor Arrays by M. R.Layton, etal, which are incorporated herein by reference.

In a fiber optic ladder sensor array, for example, a different sensor isconnected in each rung of a ladder structure having input and outputfiber optic buses connected to opposite ends of each sensor. The inputbus is connected to receive light from a single light source. Each inputlight pulse on the input bus is sequentially applied to each sensorthrough an associated directional coupler. The result is a diminution ofthe input light pulse at each sensor as the pulse progresses along theinput bus of the array, the maximum number of distributed sensors of thearray being determined by the minimum amount of light that can be passedon the output bus and detected by electronic equipment on the towingvessel. It is desirable to be able to increase the number of sensorsthat can be towed behind a vessel without increasing the number ofoptical buses required in the tow cable and without increasing theamount of processing equipment required on the towing vessel.

An object of this invention is the provision of an improved opticalfiber hyrdrophone sensor array system. Another object is the provisionof an optical fiber sensor array that requires fewer optical fiber busesthan conventional arrays. Another object is the provision of an opticalfiber sensor array or subarrays in which the optical input signal thatis passed to distributed subarrays is not reduced in intensity as itpasses the location of each subarray. Another object is the provision ofan optical fiber sensor array in which separate input light pulses ofsubstantially the same magnitude are applied to each subarray of thearray. A further object is to increase the number of sensors driven by asingle source.

SUMMARY OF INVENTION

In accordance with this invention, apparatus for sensing changes in aphysical parameter comprises: an array of a plurality of subarrays ofspaced apart optic sensors, each sensor being responsive to a physicalparameter for sensing changes therein; an input optic bus having firstand second light signals thereon; means responsive to the first lightsignal for selectively connecting the second light signal to ones of thesubarrays; and means for monitoring output signals from the sensorsubarrays for detecting changes in the physical parameter. In aparticular embodiment of this invention the first and second lightsignals are applied to opposite ends of the input bus with sensorsubarrays being spatially located along the input bus. The connectingmeans comprises a plurality of input optically actuated optic switchesin series in the input bus at associated sensor subarrays. Each pulse offirst light signal on the input bus successively actuates each inputswitch for passing to the associated sensor array a pulse of secondlight signal that is simultaneously present on the input bus at thatinput switch. Associated output optically actuated optic switches in anoutput optic bus are pulsed on in a similar manner by pulses of a thirdlight signal on the output bus for passing trains of second light signalpulses from the subarrays to processing equipment. Consideration of thisinvention reveals that it increases the number of optic sensors that canbe driven by a single optical source of second light signal since secondlight signal pulses of the same amplitude are applied to each of thesubarrays. This invention has particular utility in applicationsincluding arrays and/or subarrays of as many as 1,000 or more very smalldiameter individual sensors and microlaser sources.

BRIEF DESCRIPTION OF DRAWINGS

This invention will be more fully understood from the following detaileddescription of preferred embodiments thereof together with the drawingsin which:

FIG. 1 is a schematic block diagram of a fiber optic sensor arrayarchitecture in accordance with this invention;

FIG.'S 2A, 2B and 2C are waveforms (as a function of time) illustratinginput signal pulses S moving from right to left on input bus 14 in FIG.1, input pump pulses P moving from left to right on the same input bus14 in FIG. 1; and output pump pulses Q moving from left to right onoutput bus 16 in FIG. 1, respectively;

FIG.'S 3A and 3B are schematic representations of Mach-Zehnder opticallyactuated optic switches that may be employed as the input and outputoptically actuated optic switches, respectively, in FIG. 1;

FIG. 4 is a schematic representation of a two mode fiber optic switchthat may be employed as the input and output optic switches in thestructure of FIG. 1;

FIG. 5 is a diagrammatic representation of the operation of the systemof FIG. 1 as a function of distance x along the array (from left toright in FIG. 1) illustrating the progression of S and P pulses (inopposite directions) and Q pulses along the array at different timesthat are spaced apart by τ, where the vertical lines in FIG. 5correspond to the physical locations of switches designated in row R1;

FIG. 6 is a diagrammatic representation of the operation of the systemof FIG. 1 for noninterleaved signal puslses with distance x along thearray (left to right in FIG. 1) and time being plotted along the x and yaxes, respectively; with input signal pulses S and input pump pulses Pbeing along lines at +45 degrees and -45 degrees, respectively; withswitch locations being indicated by vertical lines 38, 39 and 40 forN=3, the interaction of successive signal and pump pulses withindividual switches producing output pulses whose center lines extendover the time intervals shown by the short dark vertical lines (e.g.52-54) on the lines 38-40; with center lines of output pulses beingshown on the left most line 75 here; and

FIG. 7 is a spatial tabulation representation of input S and P pulses oninput bus 14 and the resultant output pulses on line 16 at various timesand locations of an input pump pulse P along the input bus 14 for aninput pulse width W of ≦τ/n and n=τ=3 for interleaved signal pulses.

DETAILED DESCRIPTION

Referring now to FIG. 1, a preferred embodiment of this inventioncomprises equipment 6 located on a towing vessel and an array structure7 in a towed cable. The equipment 6 is conventional and comprisessources 10 and 11 of input optical signal and pump pulses S and P fordriving opposite ends A and B (the right and left ends in FIG. 1),respectively, of the input optical fiber bus 14. A third source 12produces optical pump pulses Q which drive the left end C of output bus16. Timing of sources 10-12 is controlled by a synchronization circuit36. Output signal waveforms from sources 10, 11 and 12 are shown inFIG.'S 2A, 2B and 2C, respectively, as a function of time. Thefrequencies of the pulses P and Q may be the same or different. Thesources 10-12 may be semiconductor diode lasers with output frequenciescorresponding to 0.8-0.9 or 1.3 or 1.9 microns, for example.

The array structure 7 comprises the input and output fiber optic buses14 and 16, a plurality of N subarrays SA_(k) and pluralities of N inputand output optical switches S_(1k) and S_(2k) connecting opposite sidesof associated subarrays to input and output buses 14 and 16,respectively, (where N is an integer, either odd or even, and kdesignates a particular switch location). The subarrays SA_(k) areconventional and are spaced apart by a distance corresponding to a timeinterval τ, as are adjacent and associated input and output switches.Each subarray comprises n sensors that are spaced apart corresponding toa delay time Δt≦τ/n. This means that 2Δt≦2τ/n is the round trip delaytime for a portion of an input signal pulse S on line 13 of a subarraysuch as SA₁ to travel through the sensor y2 and back to line 15. Thisassumes there is no delay in sensor y2. Individual sensors may beinterferometric although this is not required. In practice the dots suchas at 17 and 18 in SA₁ are directional couplers, although the directioncouplers are not shown in FIG. 1 for conveience of illustration.

Referring now to FIG.'S 2A and 2B, the input signal pulses S and inputpump pulses P are applied to opposite ends of input bus 14 (right andleft in FIG. 1, respectively) and output pump pulses Q in FIG. 2C areapplied to the left end of output bus 16. The pulses S and P have pulsewidths of W≦2τ/n which corresponds to the time delay for a signal pulseto travel from the input line for one sensor to and through and backfrom an adjacent sensor in the subarray. In contrast, the pulse width ofoutput pump pulses Q is 2τ to allow signal pulses to be emptied out ofassociated subarrays as is described more fully hereinafter. The pulserepetition frequency of pulses S in FIG. 2A is 1/τ (row R2 of FIG. 5).The pulse repetition frequency of pulses S is preferably 1/(2τ) here asis described more fully hereinafter and illustrated in row R13 in FIG.5. Alternatively, the input signal S may be a continuous wave signal,although this is not preferred since it is a waste of optical power. Thepulse repetition frequencies of the input and output pump pulses P and Qare both 1/(2T).

The lengths of the optical fibers and timing of the sources 10-12 areadjusted so that S pulses in FIG. 2A are incident on each input switchS_(1k) on input bus 14 (moving from right to left) when P and Q pulsesare applied to the first input and first output switches. This conditionis illustrated in rows R2-R4 in FIG. 5. The input switches S_(1k)operate such that when an input signal pulse S and an input pump pulse Pare both simultaneously present on input bus 14 at an input switch thenthe switch bypasses all of that signal pulse S into the adjacentsubarray and passes a substantial amount of that pump pulse P along theinput bus to the next input switch. Conversely when only an input signalpulse S is present on the input bus at an input switch (i.e., the pumppulse P is absent from that input switch) then all of the signal pulse Sis passed on the input bus through the input switch towards the next(left) input switch. The optical output switches S_(2K) are similar, inthat an output switch S₂₁ is responsive to an output pump pulse Q forpassing signal pulses from the associated subarray SA₁ on line 15, forexample, onto the output bus 16 and passing all signal pulses travelingfrom right to left on the output bus 16 to filter 30 anddetector-processor 34. The optical output switches S_(2k) are preferredover directional couplers for conserving optical power that would bewasted in the non-connected arms of the couplers.

Optically activated optical switches for switching optical signals havebeen described in the literature and are known to those skilled in theart. By way of example, the input and output switches S_(1k) and S_(2k)may be Mach-Zehnder switches of the type illustrated in FIG.'S 3A and3B, respectively, and described in the publication "Optically ActivatedIntegrated Optic Mach-Zehnder Interferometer on GaAs" by Z. Y. Cheng andC. S. Tsai, Applied Physics Letters 59(18) 28 Oct. 1991 pp. 2222-2224.Briefly, the optical switch S₁₂ in FIG. 3A comprises a pair of singlemode optical fibers or integrated optic waveguide 41 and 42 connectedbetween wavelength division multiplex (WDM) directional couplers 44 and45. The line 41 is connected in series with the straight through ports1-3 of the couplers in the single mode bus line 14. The other line 42 isconnected in series between the coupled ports 2-4 of the couplers. Theport 2 of coupler 45 has no connection to it. The port 4 of the coupler44 is connected to subarray SA₂. The signal pulses are coupled to bothlines 41 and 42. The pump pulses P are coupled to only one of lines 41and 42 to cause a change in the refractive indexes of the waveguides inthe couplers and the resulting τ phase shift and providing the desiredoperation, i.e., the requisite combining and separating of signals inthe couplers for directing a signal S pulse into port 4 of coupler 44and into the subarray SA₂. The structure and operation of the outputswitch S₂₂ in FIG. 3B is similar. Alternatively, the switches may be twomode fiber devices as is illustrated in FIG. 4 and described in U.S.Pat. Nos. 4,741,586 and 4,895,421 issued May 3, 1988 and Jan. 23, 1990,respectively, for Dynamic Couplers Using Two Mode Optical Wave guides byB. Y. Kim and H. J. Shaw which are incorporated herein by reference.Optically activated optical switches are also described in thepublications and the patents: All-Optical Modulation in Gallium ArsenideIntegrated Optical Waveguides by G. McWright, etal., SPIE Vol. 1038,Sixth Meeting in Israel on Optical Engineering (1988); Recover Time forA Silicon Waveguide All-Optical Switch, Electronics Letters 17 Mar.1988, Vol. 24, No. 6, pp. 303-305; All-Optical, Silicon Based, FiberOptic Modulator Using a Near Cutoff Region¹ by R. Normandin, etal., Can.J. Phys. 67, 412, 1989, pp. 412-419; U.S. Pat. No. 5,091,984 issued Feb,25, 1992 for Optical Switch for use With Optical Fibers by Y. Kobiyashi,etal.; Laser-Diode Pumped Non-Linear Switch in Erbium-Doped Fiber by R.H. Pantel, etal., Optics Letters Jul. 15, 1992; "Optical Kerr SwitchUsing Elliptical Core Two-Mode Fiber" by H. G. Park, C. C. Pohalski andB. Y. Kim, Optics Letters, Volume 13, No. 9, pp 776-778, 9/88;"Picosecond Switching by Saturable Absorption in a Nonlinear DirectionalCoupler" by N. Finlayson, etal., Applied Physics Letters, vol. 53, No.13, pp. 1144-1146, 9/88; "Use of Highly Elliptical Core Fibers for TwoMode Fiber Devices" by B. Y. Kim, etal., Optics Letters, Vol. 12, No. 9,pp 729-731, 9/87; "Strain Effects on Highly Elliptical Core Two ModeFibers" by J. N. Blake, etal., Optics Letters, Vol. 12, No. 9, pp732-734, 9/87, which are incorporated herein by reference.

In operation, input signal pulses S and input pump pulses P from sources10 and 11 in FIG. 1 are coupled through associated optical filters 22and 24 to opposite ends A and B of the input bus line 14. The outputpump pulses Q are coupled from source 12 through filter 30 to the leftend C of the output fiber 16. The filters 22 and 24 direct input pumpand input signal pulses P and S outputted from opposite ends B and A ofinput bus 14 to associated energy sinks 26 and 28 and away from sources10 and 11, respectively. Similarly, the filter 30 passes output pumppulses Q to output bus 16 and directs trains of output signal pulses atthe end C of output bus 16 to the detector and signal processingcircuitry 34 and away from source 12. The filters 22, 24, and 30 may beimplemented with directional couplers or other fequency selectivecomponents. Timing of the pulses is adjusted so that a signal pulse S ispresent at each input switch S_(1k) as is shown in FIG. 5, row R2, wherethe designations in row R1 are the spatial locations of the switches.

Consider now that signal pulses S on input bus 14 are present at each ofthe input switches S_(1k) (R2 in FIG. 5), that a first pump pulse P1 ispresent at the same time on input bus 14 at only switch S₁₁ (R3 in FIG.5), and that a first output pump pulse Q1 is present at the same time onoutput bus 16 at only output switch S₂₁ (R4 in FIG. 5). The pulses P1and Q1 close switches S₁₁ and S₂₁ for time intervals W=2τ/n and 2τ,respectively, for example. This causes all of the first signal pulse S1at input switch S₁₁ to pass into only the subarray SA₁. The signal pulseS1 is sequentially passed by way of coupled signal pulses S1 ofdecreased intensity through the sensors y1, . . . ,yn in theconventional manner, with a time division multiplexed output pulse trainof S1_(y1), . . . ,S1_(yn) pulses being passed by output switch S₂₁ toprocessor 34 over a time duration 2 τ. After only a time interval τ,however, the pump pulse P1 advances to and closes the second inputswitch S₁₂ (R6 in FIG. 5). This causes the signal pulse S3, now at thesecond input switch S₁₂ (R5 in FIG. 5), to be passed into and beoperated on by subarray SA₂ in the same manner. At the same time theoutput pulse Q1 is advanced down the output bus 16 and causes the secondoutput switch S₂₂ (R7 in FIG. 5) to close and pass a time divisionmultiplexed output of input signal pulses S from subarray SA₂, throughoutput switch S₂₂ and bus 16 to the processor 34. Since the output pumppulse Q1 is 2τ long, however, this means that trains of output signalpulses are simultaneously outputted from subarrays SA₁ and SA₂ throughassociated switches S₂₁ and S₂₂ over the same time interval τ onto thesame output bus 16. These pulse trains do not overlap, however, becauseof the time delay τ in the length of output fiber 16 between theadjacent subarrays (adjacent output switches) in FIG. 1. This operationcontinues with this first pump pulse P1 sequentially causing alternate(odd numbered) input signal pulses S in FIG. 2 and R2 of FIG. 5 to bepassed into consecutive switches and subarrays. Note that the signalpulse S3 is absent from R8 at S₁₁ (line 50) in FIG. 5 sincesubstantially all of S3 was bypassed by input switch S₁₂ into subarraySA₂ during the previous time interval τ (row R5).

After a time period T=Nτ the first input pump pulse P1 has progressed tothe vertical line 47 in FIG.'S 1 and 5 (R11 in FIG. 5). At this time theoutput bus 16 is filled with trains of signal pulses from the ksubarrays and the output pump pulse Q1 will still be present on outputbus 16 for an additional time interval τ (R12 in FIG. 5). This meansthat a time delay of T=Nτ is now required to empty the output bus 16before the next input pump pulse P2 is applied to input bus 14 and thefirst switch S₁₁.

Reference to FIG. 5 reveals that only alternate signal pulses S are everpassed to the subarrays SA_(k). Thus, the pulse repetition frequency ofthe signal pulses S is preferably 1/2τ in order to produce signal pulsesonly at alternate switches as is shown in R13 of FIG. 5. This causeseach input signal pulse S' to be passed to an associated subarraySA_(k). The trains of input signal pulses outputted by the subarrays areprocessed by the circuit 34 in the conventional manner for obtaininginformation about changes in parameters such as the angle of arrival andthe intensity of acoustic waves in the water.

This operation is also illustrated by the graphic representations inFIG. 6 where distance along the array is plotted along the x axis(increasing to the right) and time is applied along the -y axis(increasing downward). The center lines of input signal pulses S andinput pump pulses P are represented by the lines at +45 degrees and -45degrees, respectively; with the spacing of signal and pump pulses andthe width of signal and pump pulses being designated at the right sideof FIG. 6. The spatial locations of the switches are represented by thelight vertical lines 38-40 for N=3, with the solid vertical linesegments at 51-54, for example, representing coincidence of input signalpulses and input pump pulses at associated switches. The output signalsfrom the array are indicated on the vertical line 75 on the left side ofFIG. 6 for n=3 and n=infinity where the first and second numeralsdesignate the particular pump pulse P and switch causing the associatedoutput signal pulse train. By way of example, starting from the top ofthe FIG. 6 the signal pulse line 61 at +45 degrees and pump pulse -1 at-45 degrees are coincident at input switch S₁₃ at line 51 for producingthe output signal pulse train at -1, 3 on the left line 75. Similarly,the input signal pulses 62, 63 and 64 are coincident with the same inputpump pulse 0 at successive input switches S₁₁, S₁₂ and S₁₃ at linesegments 52, 53 and 54, respectively, for producing trains of outputpulses at 0,1 and 0,2 and 0,3 on line 75. Subsequent signal and inputpump pulses are also coincident at the same switches as is illustratedhere.

The operation of this invention will now be described analytically. Inthis description signal pulses S, P and Q are designated by lower caseletters s, p and q, respectively. The time t_(p),k for an input pumppulse p (where p=1,2 . . . ) to reach an input switch S_(1k) (for k=1,2. . . N) from the first input switch S₁₁ at k=1 is

    t.sub.p,k =[k-1+(p-1)2N]τ                              (1)

This is of the form t_(p),k =kτ plus terms that are independent of k. Itrepresents progration of a pump pulse p to the right along the inputline 14. Here k is the normalized distance along the line 14; i.e.,k=x/d where d is the distance (τ) between adjacent switches and x islinear distance along the input bus 14 at a particular point in time.From Eq. (1), the time interval τ for a pump pulse p to travel betweentwo adjacent switches such as S₁₁ and S₁₂ is

    t.sub.p,k+1 -t.sub.p,k =τ                              (2)

Also, the time interval between successive pump pulses is

    t.sub.p+1,k -t.sub.p,k =2Nτ                            (3)

where N is the maximum value of k, i.e., the total number of sub-arrays.

The time t_(s),k for signal pulse s to reach switch S_(1K) from the lastinput switch S_(1N) (where time at switch S₁₁ for K=1 is taken as areference at which t_(s),k =0) is

    t.sub.s,k =(2s-k-1)τ                                   (4)

This Eq. (4) is of the form t_(s),k =-kτ plus terms independent of k.This represents propagation of a signal pulse s to the left along theinput bus 14. The time τ for a signal pulse s to travel between adjacentswitches is

    t.sub.s,k -t.sub.s,k+1 =τ                              (5)

Assuming N=5, then from Eq. (4) it is clear that the first input pulses=1 reaches the last input switch S₁,N =S₁,5 for k=N=5 at time t₁,N=-4τ; the time for the first input pulse s=1 to reach the next to lastinput switch S₁,N-1 =S₁,4 is t₁,N-1 =-3τ; and the reference time for thefirst input pulse s=1 to reach the first input switch S₁₁ is t₁,1 =0.The time between two successive signal pulses s is

    t.sub.s+1,k -t.sub.s,k =2τ                             (6)

The time t_(p),s,k for coincidence of a pump pulse p and a signal pulses at a switch k is where t_(p),s,k =t_(s),k = t_(p),k, i.e., for valuesof s,p and k which satisfy the relationship

    s=k+(p-1)N                                                 (7)

This Eq. (7) shows that an individual pump pulse (fixed value of p)encounters N consecutive signal pulses, corresponding to the N values ofk. This Eq. (7) also accounts for repeated scanning of the array. For apump pulse p and a switch at k=N, then s=pN while for the next pumppulse p+1 and the first switch at k=1, then s=pN+1. Thus, following acollision at the last switch S_(1N) by one set of pump and signalpulses, the next collision takes place at the first switch S₁₁ by thenext successive pump and signal pulses following a time delay of Nτwhich is required for the output bus 16 to clear. Stated differently,the time difference between the arrival of the pump pulse p+1 at switchS₁₁ and the arrival of the prior pump pulse p at switch S_(1N) is, fromEq.(1),

    t.sub.p+1,1 -t.sub.p,N =(N+1)τ                         (8)

which provides the proper clearing time Nτ for output bus 16. Theclearing time may also be (N+1)τ.

Again referring to FIG. 1, in an oceanic sonic exploration applicationsuch as is described here the hydrophone sensor array 7 is towed behinda ship on which the optical sources, detectors and processing equipmentat 6 are located. This means that optical fiber buses must run betweenthe ship and the various towed arrays. It is desirable to keep thenumber of fiber buses to a minimum. In a prior art system including Nsubarrays SA₁, . . . ,SA_(N) of the type illustrated in FIG. 1, it wouldrequire a minimum of N pairs of optical fiber buses extending betweenthe ship and the arrays. In accordance with this invention in FIG. 1these N subarrays require only 3 optical fiber buses between the shipand the array 7 of N subarrays (input optical bus 14 is essentially twofiber buses since it extends from the ship to the array and back to theship). The advantages provided by this invention in reducing the opticalfiber bus count will now be illustrated analytically.

Consider that n is the maximum number of elements (sensors) in aconventional prior art subarray that can be powered from a single inputpulse. Then let a block be a contiguous or series connected group of msubarrays. This means that a block includes mn sensor elements. Considerfurther that an array contains M/mn blocks, where M is the total numberof elements in the array. In accordance with this invention only 3 fiberbuses are required for each block. This means that only 3 M/mn buses arerequired for an entire array, which reduces to 3 M/n² optical fiberbuses where m=n.

In contrast, conventional arrays (or subarrays) require one input busand one output bus for each subarray. This means that each of theaforementioned blocks requires 2 m optical fiber buses and that eacharray requires 2 m (M/mn)=2 M/n buses. This is also the number ofoptical fiber buses required when each block in a conventional squarearray of n×n (where m=n) elements.

The ratio R of the number of fiber buses required for a conventionalarray to the number of buses required for an array in accordance withthis invention is

    R=(2 M/n)/(3 M/mn)=2 m/3.                                  (9)

This means that 2 m/3 more optical fiber buses are required for aconventional array than for a similar array in accordance with thisinvention.

Consider for example that m=n=12 elements such that R=2 n/3=8. Thismeans that a conventional array requires 8 times as many optical fiberbuses as an array in accordance with this invention. Consider furtherthat there are 12 blocks in the entire array. This means that the totalnumber M of elements or sensors in the array is M=12 mn=12³ =1728, forn=m; and that a conventional array will require 2 M/n=288 optical fiberbuses whereas an array in accordance with this invention will requireonly 3 M/n² =36 optical fiber buses, which is a significant advantage.

In an alternate embodiment of this invention illustrated in FIG. 7, thepulse widths of the input signal and input pump pulses are ≦τ/n and thePRF of signal pulses is 1/τ. In this embodiment the spacing Δt in FIG. 1is made Δτ≦τ/2 n. The PRF's of input and output pump pulses P and Q arethen 1/T (i.e., no delay is required to empty output bus 16 betweeninput pump pulses) since the structure will interleave outputs ofsubarrays as is illustrated in FIG. 7 for τ=n=3.

In the tabulation in FIG. 7 time increases downward with 2 rows beingdedicated to each point in time such as at 6τ and rows R2-R3 and at 7τand rows R4-R5. The vertical lines are spaced apart a distancecorresponding to a time interval τ with input switches S₁₁ and S₁₂ andS₁₃ being located at lines at 6 and 7 and 8 here. The columns areconvenient time slot representations of the status of pulses on theinput and output buses with P and S pulses moving left to right andright to left, respectively, in FIG. 7. Row R1 shows the status of inputP and S pulses on bus 14 at the end of a time interval 5τ, where P and Spulses are simultaneously present at switch S₁₁ during the next timeinterval τ. The legend P111 designates a first pump pulse P1 at theinput switch S₁₁. Similarly, the legend S111 designates a first signalpulse S1 at the input switch S₁₁. Row R3 shows the status of P and Spulses on input bus 14 at one τ later, at the end of 6τ, with P1 and S3(i.e., P112 and S312) being coincident at input switch S₁₂. Row R2 showsthe S output pulses that are passed by subarray SA₁ onto output bus 16during the time interval 6τ. Since input pump and signal pulses are notsimultaneously present at switch S₁₂ in row R1 at this time, however,there are no S output pulses on output bus 16 in row R2 during this timeinterval. This state is represented by an x in row R2.

Comparison of rows R2 and R4 reveals that these output signal pulserepresentations are advanced to the left by one time slot τ on theoutput bus 16 during each time interval τ. The 0's in row R5, forexample, indicate that there are no signal pulses S1 and S3 on input bus14 since they were previously passed to subarrays SA₁ and SA₂ as isshown in rows R2 and R4, respectively. The second x in row R6 revealsthat there is another time interval τ at 8τ where there are no S pulsesoutputted onto the output bus 16. Row R7 reveals that input pump andsignal pulses P2 and S4 are simultaneously present at input switch S₁₁,however, at the end of 8τ. This means that an associated train of Spulses will appear on output bus 16 at the end of 9τ as is illustratedin row R8 by the pulses 41, 42 and 43 where the numeral 4 means that theoutput S pulses are caused by the input signal pulse S4 on the input busand the second numerals 1-3 mean that the pulses on the output bus areassociated with the first, second and third sensors, respectively, ofthe subarray SA₁. Thus, after a short start-up time, it is seen that thesystem interleaves output signal pulses S from various subarrays. Thisstuffing or interleaving is indicated by the dark boxes here. The signalpulses outputted on output bus 16 are deinterleaved in the processor 34in the conventional manner.

Although this invention is described in relation to preferredembodiments thereof, variations and modifications will be apparent tothose skilled in the art. By way of example, the source 20 may produce acontinuous wave (CW) output optical input signal that is applied to theend A of the input bus 14. This CW signal is converted to signal pulsesS by the operation of the input pump pulses P and the input opticalswitches S_(1k). Further, the spacing between adjacent subarrays andswitches may be 2τ such that a delay time of (N+1)τ is no longerrequired since outputs of subarrays for some pump pulses will beinterleaved with those of subsequent pump pulses. Additionally, inputand output switches may be associated with individual sensors instead ofarrays thereof. Also, different sensors may be sensitive to differentphysical parameters. Additionally optically activated optic switches maybe employed in place of the directional couplers in the subarrays suchas at dots 17 and 18 in SA₁. This invention will therefore be defined bythe attached claims.

What is claimed is:
 1. Apparatus for sensing changes in a physicalparameter comprising:means for generating first and second lightsignals; a plurality of arrays of optic sensors that are responsive to aphysical parameter for sensing changes therein; means responsive to saidfirst light signal for selectively connecting said second light signalto ones of said arrays; and means for monitoring output signals of saidplurality of sensor arrays for detecting changes in the physicalparameter.
 2. Apparatus according to claim 1 wherein said sensor arraysare passive interferometric sensor arrays.
 3. Apparatus according toclaim 1 wherein said connecting means is responsive to said first lightsignal for sequentially connecting said second light signal to differentones of said arrays.
 4. Apparatus according to claim 3 wherein saidconnecting means is responsive to said first light signal forsequentially connecting said second light signal to different ones ofsaid arrays such that pulses of second light signal of substantially thesame magnitude are sequentially connected to said arrays.
 5. Apparatusaccording to claim 4 wherein said generating means produces a firstlight signal that is a first train of first light signal pulses, saidconnecting means being sequentially responsive to the same first lightpulse over a period of time for selectively connecting at least portionsof said second light signal of substantially the same magnitude todifferent ones of said arrays.
 6. Apparatus according to claim 5 whereinsaid generating means produces a second light signal that is a secondtrain of second light signal pulses, said connecting means beingsequentially responsive to the same first light pulse over a period oftime for selectively connecting second light pulses of substantially thesame magnitudes to associated ones of said arrays.
 7. Apparatusaccording to claim 6 further comprising a transmit optical fiber; saidconnecting means comprising first means for coupling said first andsecond trains of light pulses to one and other ends of said transmitfiber; and a plurality of first optically actuated optic switchesconnected in series in said transmit fiber and each connected to aninput side of an associated sensor array, said first switches beingresponsive to first light pulses traveling in one direction on saidtransmit fiber for passing to an associated sensor array an associatedsecond light pulse traveling in the other direction on said transmitfiber when a first light pulse and a second light pulse aresimultaneously present on said transmit fiber at said first switch. 8.Apparatus according to claim 6 further comprising a transmit opticalfiber; said connecting means comprising first means for coupling saidfirst and second trains of light pulses to one and other ends of saidtransmit fiber; and a plurality of first optically actuated opticswitches connected in series in said transmit fiber and each connectedto an input side of an associated sensor array, said first switchesbeing sequentially responsive over a period of time to the same firstlight pulse traveling in one direction on said transmit fiber forsequentially passing to associated sensor arrays associated second lightpulses traveling in the other direction on said transmit fiber when afirst light pulse and a second light pulse are simultaneously present onsaid transmit fiber at said first switch.
 9. Apparatus according toclaim 8 wherein said monitoring means comprises: second generating meansproducing a third train of third light pulses; a receive optical fiber;second means coupling said third train of light pulses to one end ofsaid receive fiber; and a plurality of second optically actuated outputoptic switches connected in series in said receive fiber and eachconnected to an output side of an associated sensor array, said secondswitches being responsive to third light pulses traveling in onedirection on said receive fiber for passing from the associated sensorarray and onto said receive fiber a train of second light pulses fromthe associated array traveling in the other direction.
 10. Apparatusaccording to claim 8 wherein said monitoring means comprises: secondgenerating means producing a third train of third light pulses; areceive optical fiber; second means for coupling said third train oflight pulses to one end of said receive fiber; and a plurality of secondoptically actuated output optic switches connected in series in saidreceive fiber and each connected to an output side of an associatedsensor array, said second switches being sequentially responsive over aperiod of time to the same third light pulse traveling in one directionon said receive fiber for sequentially passing from one or moreassociated sensor arrays and onto said receive fiber trains of secondlight pulses from the associated arrays traveling in the otherdirection.
 11. Apparatus according to claim 10 wherein the pulse widthsof the first and second pulses are ≦2τ/n, where τ is the delay timebetween adjacent switches and n is an interger corresponding to thenumber of sensors in each sensor array in said apparatus.
 12. Apparatusaccording to claim 11 wherein the pulse widths of third pulses is 2τ andthe pulse repetition frequencies of first and third pulses are 1/(2T)where T=Nτ and N is an interger corresponding to the number of arrays insaid apparatus.
 13. Apparatus according to claim 12 wherein the pulserepetition frequency of second pulses is 1/(2τ).
 14. Apparatus accordingto claim 10 wherein the pulse widths of first and second pulses are both≦τ/n and the pulse repetition frequencies of first and second pulses are1/τ and 1/T, respectively, where τ is the delay time between adjacentswitches, n is an integer corresponding to the number of sensors in eachsensor array, T=Nτ, and N is an integer corresponding to the number ofarrays in said apparatus.
 15. Apparatus according to claim 14 whereinthe pulse width and pulse repetition frequency of third pulses are 2τand 1/T, respectively.
 16. Apparatus for sensing changes in a physicalparameter, comprisingmeans for generating first and second lightsignals; a transmit optical fiber having one and other ends thereofcoupled to said generating means for receiving said first and secondlight signals, respectively; a plurality of fiber optic sensors, eachsensor being responsive to a physical parameter for sensing changestherein; a plurality of first optically actuated optic switchesconnected in series in said transmit fiber, each of said first switcheshaving an output connected to an input side of an associated sensor andbeing responsive to said first light signal for connecting said secondlight signal to said sensors; and means for monitoring output signalsfrom said sensors to detect changes in the physical parameter. 17.Apparatus according to claim 16 wherein said first light signal is afirst train of first light signal pulses, said first optic switchesbeing sequentially responsive to a first light pulse traveling in onedirection on said transmit fiber for passing to the associated sensor asecond light signal that is simultaneously present on said transmitfiber at said first switch.
 18. Apparatus according to claim 17 whereinsaid second light signal is a second train of second light signalpulses, said first optic switches being sequentially responsive to afirst light pulse traveling in the one direction on said transmit fiberfor passing to associated sensors associated second light pulsestraveling in the opposite direction on said transmit fiber and that aresimultaneously present with said first pulse on said transmit fiber atassociated first switches.
 19. Apparatus according to claim 18 whereinsaid monitoring means comprises: second generating means producing athird train of third light signal pulses; a receive optical fiber havingsaid third train of light pulses coupled to one end thereof; and aplurality of second optically actuated optic switches connected inseries in said receive fiber and connected to output sides of associatedsensors, said second switches being sequentially responsive to a thirdlight pulse traveling in one direction on said receive fiber forsequentially passing from the associated sensor and onto said receivefiber a train of second light pulses.
 20. Apparatus for sensing changesin a physical parameter, comprisingmeans for generating first and secondlight signals; a transmit fiber having one and other ends thereofcoupled to said generating means for receiving said first and secondlight signals, respectively; a plurality of fiber optic sensor arrays,each sensor array being responsive to a physical parameter for sensingchanges therein; a plurality of first optically actuated optic switchesconnected in series in said transmit fiber, each of said first switcheshaving an output connected to an input side of an associated sensorarray and being responsive to said first light signal for connectingsaid second light signal to said sensor arrays; and means for monitoringoutput signals from said sensor arrays to detect changes in the physicalparameter.
 21. Apparatus according to claim 20 wherein said first lightsignal is a first train of first light signal pulses, said first opticswitches being sequentially responsive to a first light pulse travelingin one direction on said transmit fiber for passing to the associatedsensor array a second light signal that is simultaneously present onsaid transmit fiber at said first switch.
 22. Apparatus according toclaim 21 wherein said second light signal is a second train of secondlight signal pulses, said first optic switches being sequentiallyresponsive to a first light pulse traveling in one direction on saidtransmit fiber for passing to associated sensor arrays associated secondlight pulses traveling in the other direction on said transmit fiber andsimultaneously present with said first pulse on said transmit fiber atassociated first switches.
 23. Apparatus according to claim 22 whereinsaid monitoring means comprises: second generating means producing athird train of third light signal pulses; a receive fiber having saidthird train of light pulses coupled to one end thereof; and a pluralityof second optically actuated optic switches connected in series in saidreceive fiber and connected to output sides of associated sensor arrays,said second switches being sequentially responsive to a third lightpulse traveling in one direction on said receive fiber for sequentiallypassing from the associated sensor array and onto said receive fiber atrain of second light pulses.